The present invention generally relates to implantable therapeutic devices such as implantable cardioverter defibrillators that deliver therapy as a function of feedback from an implanted sensor.
Most commercially available implantable cardioverter defibrillators analyze electrocardiogram (ECG) signals to determine when to apply a stimulus to the heart to correct an aberrant rhythm. Even when the ECG is measured from the endocardium and is therefore relatively free of artifact, detection algorithms fail to perform perfectly, as evidenced by Wilkoff et. al. [Critical Analysis of Dual-Chamber implantable Cardioverter-Defibrillator Arrhythmia Detection, B L Wilkoff, et. al. Circ. 2001: 103: 381-386]
Considerable prior work and thought has been given to using hemodynamic measurements to improve detection. For example, in U.S. Pat. No. 4,774,950 to Cohen, the use of various hemodynamic measurements are proposed to improve the ability to detect Vfib and Vtach. Hemodynamic measurements previously considered viable include heart rate, stroke volume, cardiac output, blood flow and endocardial venous pressures as well as blood flow. See for example U.S. Pat. No. 5,431,685 to Alt. Much of the work in this area has been focused on the use of endocardial pressure or heart wall activity measurements, such as those obtained from the venous pressure sensing lead used on the Chronicle™“manufactured by Medtronic in Fridley, Minn. The Chronicle™”” device and method of use may be the most well developed of these techniques and involves directing a lead containing a distal pressure sensor through venous system and into the right side of the heart. However, this approach is disadvantageous because the lead occupies space within the vein that may already contain multiple endocardial leads that are used for pacing and/or defibrillation. In addition, a lead placed in the heart is subject to significant motion from the beating heart and is therefore a challenge to achieving high reliability, especially for those patients that have a long life expectancy.